WHY is my iPhone 4S not deleting my messages?

So like, basically if i delete a message thread, it deletes. Then my phone freezes, and won't allow messages in or out. I restart my phone, the threads are all there again. I've tried deleting my message threads, then leaving it overnight to let it do it's thing, it's worked once, hasn't worked since. Does anyone know any other methods? I really want to delete all my messages. It takes up a lot of space.

Try closing the app from multi tasking and repeating do this by double tap home button and swipe up if your on ios8 or earlier versions hold in and press red line